1. Field of the Invention
The present invention relates to a bearing device for machine tool spindle, and in particular, to a bearing device for a spindle of machine tool using an angular contact ball bearing.
2. Description of Relevant Art
Spindles of machine tools have bearing devices therefor, which need high rotation accuracy and, to reduce backlashes, mostly employ a plurality of angular contact ball bearings (hereafter sometimes called "angular bearing" [a bearing with a nominal contact angle between 0.degree. and 90.degree. ]) that are preloaded before use and arrayed to constitute a so-called combination bearing. For supporting spindles, typically employed is a backside combination type, as it is advantageous in that a supporting span for spindle can be set large.
For the preloading, a combination bearing includes an intervening inner collar (hereafter simply called "inner collar") between inner races of two angular contact bearings and an intervening outer collar (hereafter simply called "outer collar") between outer races of the angular contact bearings, which outer collar has an axial length different from an axial length of the inner collar so that the angular contact bearings are preloaded by the difference of length dimension. In the backside combination type, angular bearings have an increased preload, as the difference of axial length increases between an inner collar and an outer collar longer than the inner collar.
In recent machine tools, the number of revolutions per unit time (hereafter sometimes referred to as "rpm [revolutions per minute]") of a spindle is enhanced to a remarkable high speed. There are performed an increasing number of high-speed operations exceeding a 1000 rpm by far. As the spindle rpm is increased to a high speed, the influence of accompanying centrifugal forces to a bearing service life has become a significant problem.
In other words, as the rotation of a spindle is raised to a higher speed, the spindle and associated rotation members such as bearing inner races are urged radially outwards with increased magnitudes of centrifugal forces, in addition to that also balls are pushed against outer races by centrifugal forces acting thereon, with the result that the preload is increased.
Further, the spindle of the machine tool needs a sufficient rigidity to endure metal processing, and the diameter of the spindle as well as the ball weight of associated bearings is large. As a result, centrifugal forces acting on bearings of the machine tool spindle are by far larger than those acting on a small-diameter bearing of a high-speed rotation shaft in a general machine, so that the load imposed on the bearings tends to be excessive and causes a bearing service life to be remarkably reduced.
To this point, Japanese Patent Application Laid-Open Publication No. 2-279203 has proposed an arrangement in which a hydraulic piston mechanism is incorporated in a bearing section, and a hydraulic supply pressure to the piston mechanism is changed to thereby change an axial relative position of inner and outer races so that a bearing preload is changed.
Further, Japanese Patent Application Laid-Open Publication 8-200359 has proposed an arrangement in which an inner collar is made of a material that has a larger thermal expansion coefficient than a spindle, and a bearing preload under high speed rotation is reduced by the difference of thermal expansions between the inner collar and the spindle.
Further, as the spindle rotation becomes higher in speed, heat generation at a bearing portion is increased and a temperature rise in a whole spindle system is increased by the heat so that thermal deformation remarkably decreases the processing accuracy, and techniques are employed such that a bearing case, such as in the form of a spindle head body that holds an outer race, is provided with an oil jacket and a coolant is let to flow through the jacket to thereby cool the bearing portion from the outer race side.
However, in arrangements in which the bearing preload is changed by a hydraulic piston mechanism like Japanese Patent Application Laid-Open Publication No. 2-279203, the hydraulic piston mechanism has to be incorporated in a bearing section, and the number of component parts becomes large and the structure is complicated, and besides there is required a hydraulic pressure control device which may have a large scale.
On the contrary, in arrangements in which an inner collar is made of a material that has a larger thermal expansion coefficient than a spindle like Japanese Patent Application Laid-Open Publication 8-200359, the number of component parts is not increased and the structure is not complicated. But, there is employed an outer race and an inner race. The outer race is fitted in a bearing case, such as in the form of a spindle head body, with a clearance. However, as the inner race is fitted strongly tight on an outer circumference of the spindle, large frictional forces arise when the inner collar is axially displaced by a difference of thermal expansion relative to the spindle, so that the inner race is kept from slipping to make an inherent smooth displacement relative to the spindle in dependence on a thermal deformation of the inner collar, and it becomes difficult to control a varying preload in accordance with a design value.
Further, in employment of techniques for providing a cooling mechanism, such as in the form of an oil jacket, to a bearing case, such as in the form of a spindle head body, to thereby cool a bearing section from an outer race side, there arise temperature differences between the bearing case and an outer race and between the outer race and an inner race, which cause differences in thermal expansion of respective associated parts (as thermal expansions at the spindle side are larger), by which a radial clearance of the bearing becomes blocked, and heat generation at bearing section may be increased on the contrary. To avoid this, the bearing has to be assembled with backlashes intentionally given, which will reduce the accuracy of spindle rotation and the rigidity to support the spindle.